A plurality of electronic control units (ECU) is provided in a vehicle and communicates with each other to smoothly ensure various controls in the vehicle. For this purpose, a power line communication (PLC) system may be employed. The power line communication system embodies a technology that transmits signals by superimposing the signals on high-frequency carriers for communication. JP-A-2005-45327 discloses an example of the technology. According to the technology, a balanced feed line including two parallel lines is attached to a mobile object near a loop-formed coupler. This enables electromagnetic induction coupling between the mobile object and the balanced feed line.
However, the inventors have made it clear that the electric power or signal communication using electromagnetic induction coupling according to the conventional technology generates much leakage magnetic flux at portions other than a coupling portion where the electric power or signals are exchanged. Even if the above-described technique is employed, for example, when a transmission path characteristic, a surrounding environment of a system and the like change, a communication quality decreases and it is difficult to maintain a high-speed communication.